Since inkjet recording methods have advantages over other image forming methods such that the image recording process is simple; full color images can be easily formed; and high resolution images can be easily formed by an image forming apparatus having a simple structure, the inkjet recording methods have become widespread. Inkjet recording methods typically include ejecting ink droplets using an air bubble generated in the ink by heat, or using a pressure generated in the ink by piezoelectricity or electrostatic force, so that the ink droplets are adhered to a recording material such as paper sheets; and then drying the ink droplets, or allowing the ink droplets to penetrate into the recording material so that the ink droplets dry, resulting in formation of an ink image on the recording material. Thus, inkjet recording methods are simple, and can be used not only for personal purposes but also for industrial purposes such as printers and printing machines.
With expansion of industrial applications of inkjet recording, a need exists for an inkjet recording apparatus which can form images on various recording materials (such as various papers) at a high speed. Specifically, a need exists for an inkjet printer having a line recording head to form images at a high speed. In addition, an increasing need exists for an inkjet printer, which uses an aqueous ink from environmental and safety standpoint.
However, image formation using an aqueous ink is considerably affected by the recording material used, and various image problems are caused. Particularly, when a paper having a rough surface is used as a recording material, various problems are remarkably caused. Since aqueous inks have a long drying time while having good affinity for paper, aqueous inks easily penetrate into paper. Particularly, when an aqueous ink image is formed on an uncoated paper having a rough surface, the colorant included in the aqueous ink penetrates into the paper, thereby causing a problem in which a low density image is formed and which is not caused when using a solvent ink.
Particularly, when high speed image formation is performed by using an aqueous ink, a penetrant is typically included in the aqueous ink to shorten the drying time of the ink adhered to a recording material. However, when a penetrant is included in an aqueous ink, not only the permeability of water to recording materials but also the permeability of the colorant included in the aqueous ink are enhanced, thereby decreasing the image density of the resultant images more seriously than in a case where the penetrant is not used for the aqueous ink.
In attempting to solve the low image density problem, there is a proposal for an ink which include a colorant, and a crosslinking component including a urethane acrylic resin having a carbonyl group, and a dihydrazide compound. However, unless water included in the ink penetrates into a recording paper, the urethane acrylic resin is not contacted with the dihydrazide compound, and therefore the crosslinking reaction cannot be induced. Thus, at a time when the ink is adhered to a recording paper, the crosslinking reaction is not induced, and therefore the viscosity of the ink adhered to the recording paper hardly increases. Therefore, most part of the colorant penetrates into the recording paper, and occurrence of the low image density problem cannot be prevented.
In addition, there is a proposal for a combination of a black ink including a polymer dispersant having a cationic group and an anionic color ink. The cationic polymer dispersant is obtained from a cationic monomer, a hydrophobic monomer and another monomer, wherein dimethylacrylamide is exemplified as the cationic monomer, styrene or acrylate is exemplified as the hydrophobic monomer, and diacetone acrylamide is exemplified as the other monomer. In this proposal, by contacting the cationic black ink with the anionic color ink, the inks are agglomerated, thereby preventing formation of a blurred image and occurrence of penetration of the inks into a recording paper. However, when the black ink or the color ink is used alone, it is difficult to avoid the low image density problem.
In addition, there is a proposal for an ink including a colorant which is covered with a polymer having a N-substituted acrylamide unit and a carbonyl group, and a hydrazine compound, wherein dimethylacrylamide is exemplified as the N-substituted acrylamide monomer, and diacetone acrylamide is exemplified as the monomer having a carbonyl group, and wherein the ratio of the N-substituted acrylamide monomer is not less than 50% and the ratio of the monomer having a carbonyl group is not greater than 30%.
However, in this ink, the crosslinking reaction of the colorant with the hydrazine compound proceeds after the ink is dried (i.e., after water therein is evaporated). Therefore, the viscosity of the ink hardly increases at a time the ink is adhered to a recording material, and the low image density problem cannot be avoided. In addition, since the colorant covered with the polymer has poor hydrophilicity, it is necessary to introduce an anionic unit (such as acrylic acid) into the polymer in order to impart good dispersing ability to the colorant. In this case, another problem in that the viscosity of the ink increases is caused. Further, since the N-substituted acrylamide unit is altered by heat so as to be insoluble in water, the ink has poor preservation stability.
For these reasons, the inventors recognized that there is a need for an inkjet ink which has good preservation stability and which can form high density images on both a paper with a smooth surface and a paper with a rough surface.